null values

I have the following table
Amount   Category              Sub category
$5.00      Entertainment
$5.00      Groceries
$5.00      Entertainment       Entertainment-Lunch

I have a SQL table that tries to spit out a pivot like table
SELECT
CASE "Sub-Category" WHEN NULL THEN "Amount" ELSE 0 END AS "Unclassified",
CASE "Sub-Category" WHEN 'Entertainment-Lunch' THEN "Amount" ELSE 0 END AS "Lunch",
"Amount",
FROM "t_expenses" WHERE "Category" = 'Entertainment'

this should output
Unclassified     Lunch      Amount
$5.00                              $5.00
                        $5.00      $5.00

instead, I get:
Unclassified     Lunch      Amount
                        $5.00      $5.00

Is it possible to recognize a "null" Sub-category? The empty string '' did not work either.